This invention relates to superconducting solenoid coils, and more particularly to a superconducting solenoid coil which is improved in the uniformity of the magnetic field in the central operation space of the coil.
The uniformity of the magnetic field produced in the central operation space of a superconducting solenoid coil (which will be referred to hereinafter merely as a coil) has been improved by increasing the axial length of the coil or by decreasing the axial length of the coil and concentrating the magnetomotive force in both end portions of the coil. The strength of the magnetic field produced by a coil of this kind is generally chiefly dependent upon the inner diameter and outer diameter of the coil, and the coil length larger than a certain limit does not substantially contribute to an increase in the field strength but contributes merely to a better uniformity of the magnetic field. The magnetic field uniformity .epsilon. is defined herein as ##EQU1## where H.sub.z=o represents the field strength at the middle point of the central axis of the coil, and H.sub.z=2.5 mm represents the field strength at the point spaced by 2.5 mm from the middle point on the coil central axis. The magnetic field uniformity .epsilon. above described can be defined as the distribution of the magnetic field strength in the axial direction of the coil, since the field strength in a plane orthogonal with respect to the coil central axis is approximately uniform in the vicinity of the coil central axis. In order to provide a very high degree of magnetic field uniformity in a solenoid coil having such a property, it is a common design practice to concentrate the magnetomotive force in the both end portions of the coil and to make the coil length as short as possible. Such a coil is generally offered in three types. The first type called the outer notch type is provided by winding a hollow cylindrical main coil and then winding an outer shim coil on the outside of the both end portions of the cylindrical main coil. The second type called the inner notch type is provided by winding an inner shim coil and then winding a cylindrical main coil on the outside of the shim coil. In the third type, a cavity is formed within a thickness of a main coil wound in cylindrical form.
In any one of the three coil types so designed, the length of the coil must be selected to be nearly ten times as large as the inner diameter of the coil, in order that the magnetic field produced by the coil can attain a uniformity of the order of 10.sup.-6 to 10.sup.-7 within a sphere having a diameter of 5 mm formed in the middle of the coil length. The greatest defect of the coil so sized is that the proportion of the volume occupied by the coil to the volume of the space providing a high degree of magnetic field uniformity is very large, and this defect is still left as a subject to be solved.